The fast mechanical response of the mammalian auditory outer hair cell is believed to be the cellular basis of the positive feedback mechanism required for the fine tuning process of the hearing organ. We showed that the fast mechanical response of the cell is membrane potential dependent. We found that a simplified membrane model can describe the elastic property of the cell. We also found that the membrane capacitance of the cell is dependent on membrane tension as well as the membrane potential. This finding indicates that the fast motility is based on conformational changes in a membrane molecule. These observations led to a theoretical model of the cell motility, which predicts the force generation of 0.1 nN/mV for a single cell, in agreement with a value 0.2 nM/mv estimated from in vivo study. We also demonstrated that the lateral wall can also be a mechano- receptor in addition to the stereocilia. The endocochlear potential, which facilitates the mechano-electral transduction in the sensory hair cells, is attributed to the marginal cell of stria vascularis. This potential is attributable to the basolateral membrane of the cell. With the whole-cell recording method, we found that an inward current of the marginal cell is blocked by micromolar concentrations of amiloride. Patch clamp study showed that the basolateral membrane has a sodium current which is affected by amiloride. Immunocytochemical study shows the presence of amiloride channels in the cell.